The present invention relates generally to regenerative braking, and in particular, to the reduction in the amount of regenerative braking force applied to the wheels of a vehicle based on ambient temperature and a lift-throttle event.
An electric or hybrid vehicle can produce electricity and braking torque by using its traction motor as a generator during braking. This is called regenerative braking. Compression regenerative braking is applied when a driver deactivates the accelerator pedal, creating a lift-throttle event. Service brake regenerative braking is applied when the driver activates the brake pedal. Typically, total regenerative braking is determined by summing a compression regenerative braking component with a service brake regenerative component.
In vehicles employing regenerative braking and an anti-lock braking system, the regenerative braking system must work with the anti-lock braking system during an anti-lock brake event. Typically, regenerative braking is rapidly terminated in a controlled manner during an anti-lock braking event to permit the anti-lock braking system to completely control braking.
It is known to use the ambient temperature to selectively determine the amount of service brake regeneration to apply. In particular, less service brake regeneration is applied in cold temperatures to prevent slip due to a surface with a low coefficient of friction, for example, a surface with snow or ice. At higher temperatures more service brake regenerative braking is applied due to less likelihood of slip due to ice or snow.
It is also known to selectively change the rate of reduction of regenerative braking (compression and service brake regeneration) during an anti-lock braking event based on the ambient temperature. More specifically, the rate of reduction or slew-out rate of regenerative braking is increased at lower temperatures and decreased at higher temperatures. This helps maintain vehicle stability where the anti-lock brake system interprets a bump or other loss of contact with a road surface as a slip or anti-lock event.
However, selectively reducing service brake regenerative braking alone as a function of ambient temperature is not sufficient for some vehicles and conditions. In particular, a lift-throttle event that induces regenerative braking may cause slipping. This may undesirably lead to vehicle instability, especially in rear-wheel drive electric and hybrid electric vehicles.
Therefore, a need exists for a method to gradually reduce the amount of compression regenerative braking force selectively at cold temperatures where snow or ice may form on road surfaces.
The present invention provides a method to reduce the amount of regenerative braking force applied to the wheels of a vehicle when temperatures get cold enough for snow and ice to form. Preferably, an ambient temperature sensor coupled to the vehicle monitors the temperature around the vehicle. Based on the ambient temperature, a determination is made as to the amount of regenerative braking that will be applied to the wheels of the vehicle if compression regenerative braking is needed. If a lift-throttle event is detected, the selected amount of regenerative braking is applied. The amount of regenerative braking is preferably determined by selecting an amount of electrical current that is supplied from an electric motor that controls the braking force on the wheels. For temperatures above a first threshold, the applied regenerative braking force is higher than for temperatures below the first threshold. Hence at lower temperatures, less regenerative braking force is applied, which reduces the likelihood of vehicle slippage due to a road surface with a low coefficient of friction.
The selected regenerative braking force preferably varies as a function of the vehicle""s speed. This function is referred to as a map. To determine the amount of regenerative braking to be applied to the wheels of the vehicle, a map is selected based on the ambient temperature. For example, if the temperature is greater than or equal to a first threshold, a standard compression brake regen (CBR) map is selected; or, if a temperature is less than a second threshold, then a cold weather CBR map is selected. If the temperature is less than a first threshold but greater than a second threshold, then there is a linear interpolation between the standard and cold-weather CBR maps. The selected CBR map preferably determines a current that is controlled (as a function of vehicle speed) by the electric motor that is driven by the wheels.
If there is an anti-lock braking system (ABS) event, that is, a slip or lock is detected, then there is a slew out of the compression regenerative braking force to zero. In other words, the compression regenerative braking force is rapidly reduced to zero to permit the anti-lock braking system to control further braking. Reapplication of the throttle ends the application of compression regenerative braking.
A vehicle in accordance with the invention includes a temperature sensor to determine the ambient temperature, an accelerator position sensor that detects the beginning and end of lift-throttle events, and a processor that implements a stored program to perform the method described above.